PROJECT ABSTRACT Suicide is a growing health problem in the US with more than 42,000 suicide deaths and approximately 1 million suicide attempts occurring each year. Individuals identified as high risk for suicide are often referred to health systems for mental health treatment; however, there are few health service interventions known to reduce suicides or suicide attempts. Few interventions have focused on addressing hopelessness and thwarted belongingness, two risk factors for suicide emphasized by the US Surgeon General. Peer mentorship is a novel approach to addressing these risk factors. Peer mentors are individuals with a lived experience of suicidal thoughts or behaviors who have achieved stable recovery and work to support others at risk. There are over 20,000 state-certified professional peer specialists who currently provide services to many high-risk individuals and who could serve as mentors; however, no peer mentorship protocols have been rigorously studied to determine their safety and effectiveness or the barriers to implementation. PREVAIL is a peer mentorship intervention developed in a prior research study. PREVAIL consists of 3 months of one-to-one sessions between peer mentors and individuals recently hospitalized due to suicide risk, with sessions typically occurring in community settings. Peer mentors adhere to suicide safety protocols and are supervised by a mental health clinician. Peers mentors share their experiences and use semi-structured discussion guides that include content related to improving hope (e.g., setting hopeful goals, identifying reminders of hope) and belongingness (e.g., developing supportive relationships). PREVAIL has been pilot tested in a sample of 70 participants and was found to be acceptable, feasible, and without safety concerns. This study will conduct a two-site, single-blinded, randomized controlled trial of 490 adult patients admitted to an inpatient psychiatric unit for suicide risk to assess the effectiveness of the PREVAIL intervention. Participants will be randomly assigned to receive either 3 months of the PREVAIL intervention or an enhanced usual care control condition. Assessments at baseline, 3, and 6 months will measure suicidal ideation, suicide attempts, hopelessness, and belongingness. Specific Aim 1 of this study is to determine whether PREVAIL is effective at reducing the severity of suicidal ideation or the likelihood of making a suicide attempt among recipients. Specific Aim 2 is to assess the effect of PREVAIL on hopelessness and belongingness; exploratory analyses will assess whether these effects explain improvements in suicidal ideation and suicide attempts. Specific Aim 3 of the study is to identify barriers and facilitators to implementation of PREVAIL by health systems. Qualitative interviews with key stakeholders (e.g., health system leaders, peer mentors, patients) will be analyzed to guide future, timely implementation of PREVAIL. The effect of PREVAIL on outcomes relevant to health systems, such as readmissions and outpatient care, will also be explored.